Many testing and manufacturing processes require vacuum or low-pressure environments. Some of these include jet engine simulations, salt water distillation, food processing, and many chemical reactions. Steam ejectors are often used to create this low-pressure region, and can vary in size from a 0.5 in. (12.7 mm) ejector for use with fuel cells to a 40 ft. (12 m) ejector for use in metal oxidation.
An ejector is a fluid dynamic pump with no moving parts. As shown in FIG. 1 (labeled as “Prior Art”), a typical ejector 30 comprises a primary nozzle 32 and a mixing duct 34 downstream from (and generally axially aligned with) the primary nozzle 32. The ejector 30 uses a high velocity core flow 36, typically air or steam, to entrain a secondary, ambient flow 38, which can be a gas, liquid, or liquid/solid mix. In operation, the high velocity core 36, moving in the direction indicated, creates a low pressure region 40 which sucks in the ambient flow 38. As a result, the primary and secondary flows mix to an extent, and the pressure increases and then reaches ambient conditions at the exit end of the mixing duct 34. Ejectors can be used as pumps (i.e., specifically for moving the secondary flow), or they can be used for purposes of creating low-pressure or vacuum regions (moving the secondary flow reduces the pressure upstream from where the secondary flow is drawn into the mixing duct). The key performance factor for suction ejector systems is the vacuum they can generate while pumping a required load (secondary flow).
A supersonic steam ejector system, an example of which is shown in FIG. 2 (labeled as “Prior Art”) is a relatively common type of ejector system that operates at extremely high pressure. The steam ejector system 42 uses a choked, converging/diverging, round primary nozzle 44 in conjunction with a convergent/divergent diffuser or ejector 46 (acting in place of a mixing duct 34). In operation, once a primary steam flow  48 leaves the nozzle 44, it supersonically expands out to the area of the diffuser 46. The primary flow then mixes with the entrained secondary flow 50. The mixed flow then passes through the diffuser 46, which reduces the flow's velocity and increases its pressure by the time the flow reaches the diffuser exit, with the higher the exit pressure, the lower the energy lost. For this purpose, the diffuser 46 has three regions: a supersonic diffuser portion 52 with a converging cross-sectional area; a throat portion 54 with a constant cross-sectional area; and a subsonic diffuser portion 56 having a diverging cross-sectional area.
The problem with steam ejector systems is that they are very expensive to fabricate and operate. More specifically, because a long mixing region is needed, the length of the diffuser 46 is very long—oftentimes 3 ft. (1 m) or more. This results in significant material and manufacturing costs. Moreover, the high-pressure steam jet required to produce the vacuum results in high operational costs. These problems are compounded where multiple steam ejector systems are put in series to increase vacuum capability.
Accordingly, it is a primary objective of the present invention to provide a significantly shortened, less expensive air or steam ejector vacuum system with improved vacuum/pumping performance.